Identification and analysis of type II TGF-β receptors in BMP-9-induced osteogenic differentiation of C3H10T1/2 mesenchymal stem cells.

Our previous studies have demonstrated that bone morphogenetic protein 9 (BMP-9) is one of the most efficacious BMPs to induce osteoblast differentiation of mesenchymal stem cells (MSCs). However, the molecular mechanism underlying the BMP-9-induced osteogenic differentiation of MSCs remains to be fully elucidated. In this study, dominant negative (DN) type II TGF-β receptors were constructed and introduced into C3H10T1/2 stem cells, then in vitro and in vivo assays were carried out to analyze and identify the type II TGF-β receptors required for BMP-9-induced osteogenesis. We found that three DN type II TGF-β receptors, DN-BMPRII, DN-ActRII, and DN-ActRIIB, diminished BMP-9-induced alkaline phosphatase (ALP) activity, led to a decrease in BMP-9-induced Smad binding element (SBE)-controled reporter activity, reduced BMP-9-induced expressions of Smad6 and Smad7, and decreased BMP-9-induced mineralization in vitro and ectopic bone formation in vivo, finally resulted in decreased bone masses and immature osteogenesis. These findings strongly suggested that three wild-type II TGF-β receptors, BMPRII, ActRII and ActRIIB, may play a functional role in BMP-9-induced osteogenic differentiation of C3H10T1/2 cells. However, C3H10T1/2 stem cells can express BMPRII and ActRII, but not ActRIIB. Using RNA interference (RNAi), we found that luciferase reporter activity and ALP activity induced by BMP-9 were accordingly inhibited along with the knockdown of BMPRII and ActRII. Taken together, our results demonstrated that BMPRII and ActRII are the functional type II TGF-β receptors in BMP-9-induced osteogenic differentiation of C3H10T1/2 cells.